Hand held portable firearms including pistols, revolvers, rifles, light machine guns and ammunition used with these small arms have existed for many years. Ballistics is the science that deals with projectiles in motion, including their terminal performance upon target strike. Ballistics can be divided into four main categories: interior, intermediate, exterior and terminal ballistics. Interior ballistics covers all processes from the point of primer strike to that instant at which the projectile just clears the muzzle. Intermediate ballistics examines all near muzzle blast field effects and interactions with the projectile, whilst exterior ballistics is concerned solely with the projectile's region of free, uninterrupted flight. Terminal ballistics describes the projectile's interactions with the target. Targets are generally classified as being either “soft” or “hard”. Soft targets include living creatures including animals and people and hard targets are typically man made objects such as buildings, vehicles and general defense materiel. In many situations, hard targets can protect soft targets, for example, a person within an armored vehicle or wearing protective body armor.
For optimal performance, different types of projectiles are used for hard and soft targets. Hard target projectiles are designed to penetrate through the hard protective materials in order to disrupt the critical internal components of the target. In contrast, soft target projectiles are designed to enter and deposit all of their energy within the body in order to maximize the terminal effect. A projectile that enters and exits a soft target may not cause substantial injury if critical organs and structures are not encountered along the projectile's trajectory through the body.
With reference to FIG. 1, the projectile 109 is coupled to other components to form a cartridge 101. The cartridge 101 components include: the head 111, cartridge casing 103, primer 105, propellant 107, and projectile 109. The cup 102 is a circular metal structure attached to the base of the cartridge casing 103 that is a thin metal cylindrical structure. The primer 105 is a highly sensitive pyrotechnic composition that is stored in the cup 102. The propellant 107 is a combustible energetic material that is stale in storage and transportation, but burns rapidly into gaseous combustion products when ignited. The propellant 107 is contained in the cartridge casing 103. The front of the cartridge casing 103 forms a circular mouth having an inner diameter. The projectile 109 or bullet is a solid, hollow or composite metal structure that typically has a cylindrical body with a pointed nose. The projectile 109 is an interference fit into the inner diameter of the mouth of the cartridge casing 103. The projectile may also be crimped into the mouth of the cartridge casing. The coupling of the projectile 109 to the casing 103 holds the cartridge 101 components together.
To launch the projectile, the cartridge 101 is placed in the chamber of the small arm. The interior ballistics begin when the user activates the trigger, thereby enabling the firing pin to strike the primer 103 causing it to ignite. The burning primer 103 emits a stream of hot gases and incandescent particles that contact and initiate the burning of the propellant 107. As the propellant 107 granules burn they are converted into expanded gaseous products. The pressure from the expanding gas exerts a force across the base area of the projectile 109, causing it to separate from the mouth of the casing and travel through the bore of the firearm. The instant the projectile first begins to move is known as the “shot start” time. The outer diameter of the projectile 109 has a bearing surface that contacts the rifling form within the bore. The contact with the rifling engraves the projectile 109 causing it to rotate axially within the bore. The in-bore frictional forces associated with engraving have a significant bearing on back-pressure, propellant 107 burning rate profile and peak chamber pressure within the firearm.
The “all burnt” point occurs when all propellant granules are said to have been consumed and may occur when the projectile has traveled about one half to two thirds of the length of the barrel. An early all-burnt point may result in a greater thermal efficiency with more of the available chemical energy converted into projectile kinetic energy and less energy appearing as light, heat and blast. Increased thermal efficiency may cause higher muzzle velocities, higher peak chamber pressures and mechanical loads on the firearm.
Intermediate ballistics examines all near muzzle blast field effects and interactions with the projectile. As the projectile 109 exits the barrel, the near muzzle blast overpressure may typically be about 15% of the peak chamber pressure. The flow of high pressure gaseous products out of the muzzle continues to act on the base of the projectile 109 for a short time, such that the projectile's 109 maximum velocity occurs at a short distance after it has it has left the barrel. The projectile 109 then enters a region of free flight known as exterior ballistics, on its way to target strike. The time interval between shot start and target strike is the time of flight.
With reference to FIG. 2, a projectile 109 is shown separated from the cartridge. Small arms projectiles usually consist of a nose section or forebody 113, a cylindrical bearing section or midbody 115 which is engraved into the bore and if it has one, a tapered rear section known as the afterbody 117. The projectile's forebody 113 curvature may be described in terms of either a conical, tangent, secant or complex ogive.
The midsection 115 may feature some form of circumferential groove or cannelure 119 that improve the crimped connection with the mouth of the cartridge casing. While the crimped coupling is a desirable precaution for overcoming de-bulleting in auto and semi-automatic weapons, it also ensures good, consistent backpressure for efficient propellant burning. Small arms projectiles can include a core comprised of lead doped with antimony, all of which is encased in a gilding metal jacket typically comprising 5-10% zinc with the balance as copper. Alternatively, the small arms projectiles might be comprised of unjacketed lead, doped with antimony to improve “stiffness”.
The projectile may have design features that improve the exterior ballistic aerodynamic efficiency. For example, the projectile 109 may have a tapered or boat tailed afterbody 117 that is smaller in diameter than the diameter of its midbody 115. The tapered afterbody causes air flowing over the projectile 109 to converge rapidly into the low pressure drag region behind the afterbody 117. By decreasing the projectile's 109 base drag, the projectile's total drag is reduced and the aerodynamic efficiency is improved in all regions of flight: supersonic, transonic and subsonic. Efficient boat tails may typically have a taper angle of approximately 7 degrees.
The terminal ballistics begin when the projectile strikes the target. At contact, the projectile may completely perforate and pass through the target, penetrate and remain within the target, or strike the target surface without breaching the surface structure. The terminal ballistics will depend upon the design of the projectile and the nature of the target. There are different types of projectiles that are used for different applications including homogeneous projectiles and jacketed projectiles. Homogeneous projectiles are constructed from a single piece of material such as annealed solid copper projectiles and yellow brass projectiles. The main advantage of homogeneous projectiles is that they have virtually no asymmetry in their radial mass distribution, that is, all else being equal, they are effectively perfectly balanced projectiles that can withstand extremely high angular velocities if required. This axial rotation is typically quantified in units of revolutions per minute.
Another type of projectile is the jacketed projectile which uses high hardness steel cores or tungsten/alloy cores that are placed within a gilding metal jacket for greater penetration of hard targets. These “full metal” jacketed projectiles are typically used by the military. As an alternative to alloy cores, the jacketed projectiles may also have composite cores having a plurality of core components. An example of a jacketed projectile having a composite core is Fabrique Nationale's 5.56 mm×45 SS109, otherwise known as NATO's second small arms standard as defined by STANAG 4172. The SS109 projectile has three core components: an air-space behind the nose cone, a high hardness truncated conical steel penetrator in the forebody and a lead/antimony base core. The core components are housed within a gilding metal jacket.